The present invention relates to an electrical insulating oil and more particularly, to an electrical insulating oil which has high thermal stability and also exhibits excellent gas absorption characteristics and streaming electrification characteristics.
In recent years, transformers with super high voltages (higher than 1 million volts) which are of large size are being developed to meet with an increased demand for electric power.
Accordingly, in order to obtain an improvement of insulating reliability, the required properties for electric insulating oils are becoming more severe. Particularly, in insulating oils for super high voltage transformers, an increment of dielectric loss tangent (tan .delta.) accompanied by deterioration of thermal stability is becoming a serious problem. The heat generated in a dielectric is increased with increased tan .delta., which results in heating of the transformers.
This is a vital problem for an electric insulating oil composed mainly of a naphthene-based lubricant oil fraction when used as an electrical insulating oil because it is poor in stability and its dielectric loss tangent (tan .delta.) changes markedly.
Various causes are considered for an increase in tan .delta.. It is reported that the presence of copper is responsible for such an increase in tan .delta. although it has not yet been clarified. Thus an attempt to control an increase in tan .delta. by adding a small amount of 1,2,3-benzotriazole (B.T.A.) quantitatively reacting with copper to the usual mineral oil base oil has been made. But, it is difficult to keep down the increment of tan .delta. completely, and moreover, an introduction of an improved technique has been hoped for.
The present inventor have already succeeded in developing an electric insulating oil which can solve the above problems and is substantially decreased in a change of tan .delta. with time (U.S. Pat. No. 4,584,129).
For an electric insulating oil to be used at a high or ultra-high voltage, streaming electrification is necessary to be small because a danger of discharge due to streaming electrification is higher than at the usual voltage. Furthermore there is a danger that the formation of gases resulting from decomposition of the insulating oil itself is accelerated. Thus it is required for the insulating oil to be excellent in gas absorption characteristics and streaming electrification characteristics. Furthermore, it is required for the insulating oil to be excellent in dielectric breakdown voltage at a low temperature.